The invention relates to the casting of metals, e.g. steel. In particular, the invention relates to continuous casting machines having a plasma torch for heating the metal as the metal is passed from place to place in the tundish.
In a continuous casting system, the liquid steel contained in the casting ladle where its composition is adjusted is not directly teemed into the bottomless mold having cooled walls, in which molds solidification is initiated and carried out. Rather, the metal is first passed into a container designated a tundish (or distributing container) which has a refractory interior lining. The tundish has a number of functions. Firstly, one or more openings, called tundish nozzles, are provided in the bottom of the tundish. Each such opening is disposed over a respective mold. In this way, liquid metal can be distributed to a plurality of molds even though the casting ladle has only one outlet opening for the metal. Secondly, the tundish serves as a reservoir of liquid metal which allows casting of the metal to continue after the ladle is emptied, during the time the empty ladle is being moved away and replaced by a new ladle and the teeming of metal from the new ladle is begun. In this way, continuous casting can be conducted without interruption using the contents of a whole series of successive ladles, which process is called "sequential continuous casting". Finally, the tundish advantageously serves as a container for the decantation of undesirable non-metallic inclusions present in the liquid steel; the higher the mean residence time of the metal the more important such a capability is.
In certain continuous casting facilities it is possible to affect the temperature of the liquid steel by means of a heating device. This capability affords certain advantages:
One can reduce the range of variability of the temperature of the steel leaving the tundish during a casting operation. Generally the time to empty a single ladle is on the order of tens of minutes, during which time the temperature of the liquid steel contained in the ladle may drop by tens of degrees centigrade. Particularly near the end of the casting of a given heat, the ability to add energy to the contents of the tundish allow one to compensate at least partially for such temperature decreases. By such appropriate such heating one can limit variations of the temperature of the metal leaving the tundish to a range of only several degrees over the entire casting operation.
The temperature of the metal in the earlier refining stages can be reduced, with resulting gains in the productivity and economic efficiency of the steelworks. E.g., the heating times for the metal during converter treatment, and/or in an electric furnace or furnace-ladle, can be decreased, and savings can be achieved by the reduced erosion of refractory materials lining the various metallurgical vessels.
In general, this tighter control of the temperature makes it easier to obtain a temperature of the steel in the tundish which is relatively close to the liquidus temperature of the alloy being cast. The difference between the two temperatures is called the "superheat".
From a metallurgical standpoint, a low superheat favors the production of a solidified product which has a low degree of segregation of alloying elements over the cross section of the product--such elements as carbon, manganese, and sulfur; accordingly, such a product has good homogeneity of mechanical properties. Such homogeneity is particularly important in casting of high alloy steels. Further, a low superheat allows a short solidification time for the product, and thereby a higher speed of casting, resulting in improved productivity of the steelworks; it also allows one to devise a continuous casting machine of more compact dimensions, resulting in savings in invested capital.
A first means of supplying thermal energy to metal passing through the tundish is to pass at least part of the metal through a channel surrounded by an inductor having suitable characteristics, wherewith the currents induced in the metal will cause heating by the Joule effect. Such a technique is costly, and the substantial space required by the inductor system makes the technique difficult to employ in installations of small dimensions or installations not originally designed for use with induction heating.
Another heating means consists of mounting one or more plasma torches above the liquid metal in the tundish. PCT application WO 95/32069 describes a tundish thus equipped. The reader will recall that a plasma torch operates essentially by introducing a pressurized gas (a plasmagenic gas, such as nitrogen or argon) above the material to be heated. This gas is caused to pass over an arc generated between a cathode and an anode, whereby the gas is partially ionized and is brought to a very high temperature (4,000 to 15,000 K). The hot gas has a high thermal conductivity and high radiative power, rendering it capable of transferring heat rapidly and intensely to the material to be heated. By varying the pressure of the gas and the intensity of the current, one can easily achieve the power levels needed to obtain the desired heating of the steel in the tundish, namely several hundred kW. At the same time, suitable plasma torches are small enough to be used in tundishes of relatively compact size.
Two different types of plasma torches may be used in the described application. The first type, the "propelled plasma" torch, has both cathode and anode built into the torch. In the second type, the "transferred plasma" torch, only the cathode is built into the torch. The anode is comprised of the liquid metal to be heated, and an electrically conducting element is provided in the bottom of the tundish, which conducting element contacts the liquid metal during the casting operation and is connected to the positive terminal of the electric power supply of the torch. Alternatively, in a "transferred plasma" torch, the anode may be built into the torch and the cathode may be provided in the bottom of the tundish.
The zone of the tundish in which the torch is mounted should be enclosed by a cover having a refractory interior lining. This cover prevents exposing personnel walking in the vicinity of the apparatus to the intensely bright radiation from the arc. Further, the liquid metal under the torch, upon which the torch acts, must be bare, and in particular cannot be covered by the thermally insulating powder which is customarily spread over the liquid metal surface so as to protect the liquid metal from oxidation by the atmosphere and to stop radiation emitted by the liquid metal. In addition to the plasmagenic gas, one may introduce an inert gas such as argon under the cover (or during periods when the torch is not being used one may introduce the inert gas instead of the plasmagenic gas). This allows the atmosphere in the neighborhood of the torch to be kept practically free of oxygen which could otherwise tend to cause contamination of the liquid metal.
A substantial amount of the radiation from the arc emitted by the torch impinges on the refractory materials which line the tundish and the cover of the tundish. Consequently, said refractory materials are brought to a very high surface temperature which may exceed 1800.degree. C. when the torch is operated at high power. At such temperatures, magnesia and alumina, which are the refractory materials customarily used, approach their fusion points; the linings deteriorate rapidly, and require frequent replacement, particularly the lining of the cover. Moreover, refractory material which has been fused tends to flow or drip onto the surface of the metal bath, where it forms an insulating crust which impedes heat transfer between the plasma and the metal and which eventually may cause the arc to be extinguished in the case of a "transferred plasma" torch. Fused refractory material may also flow or drip from the cover onto the metal sheath surrounding the torch, damaging the sheath. Consequently, it becomes necessary to find an operating regime of the torch which is a compromise between insufficient heating of the metal and excessive deterioration of the refractories; such a regime (if it exists) comes at a cost to the optimum efficiency theoretically available with the use of a plasma torch.
One way to solve the problem is to line the tundish and cover with a refractory material having a higher fusion temperature than materials customarily used; e.g. one might use silicon carbide or a ceramic. However, regardless of the lining material used it is necessary to replace the tundish lining after every casting operation or sequence of casting operations. The use of a higher grade refractory will thus substantially increase the operating costs of the apparatus, canceling out most of the economic advantage of using a plasma torch.
The object of the present invention was to devise economical means of limiting the deterioration of the refractory lining of a tundish and tundish cover in the zone of action of a plasma torch, without compromising the energy efficiency and economic efficiency of using a plasma torch for heating the metal.